Cold crucible induction melting is widely used for melting and forming reactive metals having high melting points, such as titanium, zirconium and the like. In most induction melting processes, a crucible of a refractory material, such as aluminum oxide, is used to contain the metallic charge. However, high melting point and reactive metals, such as titanium, zirconium, hafnium, molybdenum, chromium, niobium and other metals and alloys of that type cannot be melted successfully in refractory crucibles. When molten, such metals react with and dissolve refractory crucibles, causing the melt to become contaminated.
The solution to the contamination problem has been to cool the crucible to avoid temperatures high enough for reactions to occur between the crucible and the contained metal. This solution relies on crucibles made usually of copper and cooled by circulating water through cooling passages inside the crucible walls and bottom. So-called "cold crucibles" are typically constructed from metals having high thermal conductivity, such as copper, and are cooled, typically by circulating water, in order to hold the temperature of the crucible below temperatures at which reactions between the crucible and the metal being melted would occur. Cold crucibles of this type are disclosed in U. S. Pat. Nos. 3,775,091, 4,058,668 and 4,738,713, and in United States Department of the Interior Bulletin 673, entitled "The Inductoslag Melting Process," by P. G. Clites (1982).
Without exception, the induction coils used with the cold crucibles known in the art do not extend past the top of the crucible. That is, the entire coil is below the plane defined by the top of the crucible. The primary reason for this is to enable the metal in the crucible to be poured out into molds for casting. At the end of the melt cycle, the crucible is tilted and the metal is poured into one or more molds. The induction coil is tilted with the crucible, and the coil is kept below the top of the crucible so that metal will not contact the coil during pouring.
The problem with cold crucible induction furnaces of this type is that very little of the induction field generated by the induction coil is able to get through the crucible walls to the metal inside the crucible. This means that the cold-crucible induction melting process is very inefficient.
It is an object of the present invention to provide a cold-crucible induction furnace with improved coupling of the induction field from the induction coil to the metal contained in the crucible and therefore improve significantly the efficiency of the cold-crucible induction melting process. However, it should be understood that the present invention, while especially effective in improving the efficiency of the cold-crucible induction melting process is not limited to that process, and can be used in all types of induction melting and heating where increased efficiencies are desired.